Lubricating oils used in passenger cars and heavy duty diesel engines have changed over the years. Today's engines are designed to run hotter and harder than in the past. Various additives have been added to lubricant formulations in order to reduce friction between moving parts. One particularly common additive is the organo-molybdenum additive. While such molybdenum additives are particularly useful as friction modifiers, such molybdenum friction modifiers may have one or more of the following disadvantages: poor oil solubility; copper and/or lead corrosion; color darkening of the finished lubricant; and increased levels of sulfur and/or phosphorus in the finished lubricant.
Future generations of passenger car motor oils and heavy duty diesel engine oils require lower levels of phosphorus and sulfur in the finished oil in order to protect pollution control devices as it is well known that sulfur and phosphorus containing additives may poison or otherwise reduce the effectiveness of pollution control devices. For example, current GF-4 motor oil specifications require a finished oil to contain less than 0.08 wt % and 0.7 wt % phosphorus and sulfur, respectively, and PC-10 motor oil specifications, the next generation heavy duty diesel engine oil, requires oils to contain less than 0.12 wt % and 0.4 wt % phosphorus and sulfur, respectively, and 1.0 wt % sulfated ash. Certain molybdenum additives known in the industry contain phosphorus and sulfur at levels which reduce the effectiveness of pollution control devices.
Therefore, a need exists for lubricant additives and compositions that provide enhanced friction reducing properties and which are more compatible with pollution control devices used for automotive and diesel engines. A need also exists for such lubricant additives and compositions which are more compatible with such pollution control devices without adversely affecting oil solubility, corrosion, and/or darkening the color of the finished lubricant. Such additives may contain phosphorus and/or sulfur or may be substantially devoid of phosphorus and/or sulfur.